This application claims the benefit under 35 U.S.C. 371 of PCT/EP99/00341 filed Jan. 20, 1999.
The present invention relates to a process for producing close-to-final-shape mouldings from moulding compositions, in which reinforcing fibres, fibre bundles, fibre fabrics, fibre mats and/or random fibre agglomerates based on carbon, nitrogen, silicon and/or boron, one or more binders and, if desired, one or more additives or fillers are mixed to give a pressing composition and are pressed in a pressing mould under the action of pressure to give a moulding, where a curable carbonizable polymer is used as binder.
Such mouldings are also referred to as green bodies. They serve, for example, as intermediates within the overall process for producing C/SiC ceramic parts, as is known, for example, from DE 44 38 455 C1 and from the applications 197 11 831.3 and 197 11 829.1 which are not prior publications.
In these known processes, conditioned fibres or fibre bundles which have generally been coated with pitch and/or carbonizable polymer are mixed with a binder and, if desired, fillers or further additives and pressed to give a close-to-final-shape green body. The binder is cured at temperatures above 120xc2x0 C., generally at from about 140 to 180xc2x0 C. (hot curing). The green body is pyrolysed under protective gas to form a porous C/C body, with the binder being carbonized to give a carbon matrix. The porous C/C body is subsequently melt-infiltrated with liquid metal or liquid silicon. Here, the silicon reacts in large measure with the carbon matrix to form ceramic silicon carbide.
The pressing procedure determines important materials properties of the future C/SiC ceramic part. The greater the pressing pressure or the longer the pressing time, the more dense the resulting green body and the smaller the future pore content. It follows from this that the metal or silicon content of the ceramic body is also low after the melt infiltration. These properties are desired, in particular, in the production of brake discs. The desired density of the green body depends essentially on the starting material. It can be, for example, in a range from about 1.2 to 1.5 g/cm3, which results in a correspondingly low silicon content in the finished C/SiC ceramic part. The pressing composition or the resulting part therefore has to be subjected to a high pressure within the mould during curing. Since the pressing composition is compressible, sometimes considerable amounts escape at the joints of the mould during hot curing of fibre-reinforced pressing compositions having a high proportion of binders (up to 50% by weight based on the fibre content) because of the pressing pressure (and sometimes because of softening of the binder).
The binder which has escaped and is located in the joints and outside on the mould is likewise cured and presents problems in the rapid removal of the finished green body from the mould. Furthermore, the contaminated mould has to be subjected to cumbersome and time-consuming cleaning to remove the residues of resin which have escaped. This leads to time delays in production, which is disadvantageous, particularly in mass production.
It is therefore an object of the present invention to improve a process of the abovementioned type in such a way that the disadvantages mentioned are encountered to a very small extent, if at all, especially in mass production.
The solution is to use a cold-curing binder and to initiate the curing reaction by addition of a catalyst, with pressure only being applied after the curing reaction has commenced.
The feature xe2x80x9ccold-curing binderxe2x80x9d is intended to encompass all substances which cure at temperatures up to about 100xc2x0 C.
The process of the invention has the advantage that a transitional, thermally induced lowering of viscosity of the binder does not occur, but rather the binder becomes steadily more solid during the course of curing and the pressure can also be increased to this degree without binder residues getting into the joints of the mould or even escaping to the outside at these joints. A further advantage is the energy saving, since no heating is required. Furthermore, the catalytically initiated curing reaction proceeds more rapidly and if the reactants and the catalyst are paired appropriately even occurs xe2x80x9csuddenlyxe2x80x9d after an induction phase. For this reason too, binder residues cannot penetrate into the joints of the mould. The rapid curing reaction also leads, particularly when an appropriate binder system is chosen, to the resulting green body being strong and dimensionally stable and not being subject to springback on opening the mould as a result of the compressibility of the reinforcing fibres.
Any loss in strength which nevertheless occurs because of the late application of pressure, i.e. in the case of the curing reaction having already progressed to a considerable extent, can be accepted because of the high total binder content made possible according to the invention.
Advantageous embodiments are described in the subordinate claims.